Electron beam injector and focusing means suitable for electron microscope

ABSTRACT

An electron beam focusing means including a grid cap and accelerating lenses is provided. To permit a small change of voltage on the grid cap to overcome the defocusing effect of a great change in voltage on the accelerating electrodes, a short focal length accelerating lens is positioned in the front focal length of the long focal length lens constituted by the high voltage accelerating electrodes.

United States Patent [72] Inventor John W- Coleman [50] Field of Search313/81, 82, Willingboro, NJ. 82 T; 315/14, 15, 31; 250/495 (3), 49.5(7), 49.5 [21] Appl. No. 759,431 (9) [22] Filed Sept. 12, 1968 [23]Division of Ser. N0. 577,353, Sept. 6, 1966, R f r n s Cite P d 8 fUNITED STATES PATENTS atente c 2,617,060 11/1952 De Gier 313/82T [73]Assgnee f 2,911,563 11/1959 Atti et a1 313/82 T x 3,100,260 8/1963Wilska 250 495 3 Primary ExaminerMalcolm F. Hubler Attorney-Markva,Smith & Kruger [54] ELECTRON BEAM INJECTOR AND FOCUSING MEANS SUITABLEFOR ELECTRON AlZSTRACT. An electron beain focus ng means including aMICROSCOPE grid cap and accelerating lenses 1s prov1ded. To permit asmall 6 Claim 1 D in Fi change of voltage on the grid cap to overcomethe defocusing raw g effect of a great change in voltage on theaccelerating elec- [52] US. Cl 315/15, trodes, a short focal lengthaccelerating lens is positioned in 250/495 R, 313/82 R the front focallength of the long focal length lens constituted [51] Int. Cl HOlj 29/58by the high voltage accelerating electrodes.

CIJTHODE 40 VOLT. o l 10 16 x 14 7,6000 TC car-H005 10 VOLT. To CQTHODE10 0 voLr. TO CFTHODE 10 VOLT. 7'0

LIZWfli/d awn/a0: 10 o JELO/YD Lf/VS +56KV639KV A l M r0 CHTHODE 1oELECTRON BEAM INJECTOR AND FOCUSING MEANS SUITABLE FOR ELECTRONMICROSCOPE This application is a division of application Ser. No.577,353, filed Sept. 6, 1966 now U.S. Pat. No. 3,452,241.

This invention relates to an electron beam producing means, for example,an electron gun which produces a collimated electron beam that isparticularly suitable for use in an electron microscope but is notlimited to that use.

When an electron beam produced by a beam producing means or gun isaccelerated, as by accelerating electrodes, the voltage applied to theaccelerating electrodes has a lens effect on the beam, whereby a changein accelerating voltage requires a change in focusing of the beam.

It is an object of this invention to provide an electron beam source fora linear electron accelerator in which defocusing of the beam by changesof acceleration voltage is minimized.

In accordance with this invention, lens means are provided for focusingthe beam and means are provided for accelerating the electrons thereof,the lens means and the accelerating means including an element incommon. While the electron beams so provided can be used for manypurposes for which an electron beam gun is suitable, such as the beamgun of a cathode-ray tube, it can also be used in a high-voltage beammicroscope with further beam accelerating means. When so used, thedefocusing of the beam resulting from changes in voltage of the furtheracceleration means is minimized by the construction of the electron gun.

The invention may be better understood upon reading the followingspecification in connection with the accompanying drawing in which thesole FIGURE is a fragmentary section of an election microscope takenthrough a vertical axis thereof.

Turning to the FIGURE, a filamentary cathode 10 which is heated and heldat a high negative accelerating potential with respect to ground bysuitable means, not shown, acts as a source of electrons for the beam12. A grid cap 14 which is held at a bias voltage such as 45 volts withrespect to the filamentary cathode l surrounds the cathode 10. The gridcap 14 is symmetrical about its central vertical axis as viewed in theFIGURE. The central portion of the cap 14 is in the form of a plate 16having a hole 18 therethrough for the passage of electrons from thecathode l0, and the outer portion of the grid cap 14 is formed toprovide an outwardly and downwardly slanting conical wall 20. A furtheraxially symmetrical electrode 22 which comprises an annular-shaped disk24 and a cylindrical portion 26 is positioned coaxially with the cap 14and with the disk 24 below the cap 14, the cylindrical portion 26surrounding the grid cap 14. The electrode 22 is maintained by suitablemeans, not shown, at the voltage of the cathode 10. An axiallysymmetrical electrode 28, which is maintained at about 6,000 voltspositive with respect to the cathode by suitable means, not shown, isalso positioned coaxially with the cap 14. The electrode 28 comprises ahollow portion 30 which is frustoconical both at its outer and innersurfaces, the top of the portion 30 extending up into the conicalportion 20 of the grid 14. The upper end of the portion 30 has a holetherethrough for passage of electrons. The lower part 32 of the conicalportion 30 is cylindrical in internal shape. The conical portion 30 isjoined to an annular disklike portion 34 by an intermediate portion 36.The lower edge of the intermediate portion 36 is hollowed out as shownto provide clearance between the electrode 28 and an electrode 38 aswill be more fully explained.

Due to the shape of the lower surface of the grid cap 14 and to theshapes of the electrodes 22 and 28 and due to the potentials appliedthereto, the beam of electrons l2 exiting through the hole 18 willexpand radially in a symmetrical manner as it goes down and willtherefore take the conical shape shown, the cylindrical portion 32causing a speeding up of the radial expansion of the beams since thesurface 32 does not recede from the beam as does the internal conicalsurface portion just above it. This expanding portion of the beam 12acts as if it originated at the point 40, at the apex of the linescomprising the extensions in an upward direction of the envelope of thebeam 12. Means are provided to hollow out the beam 12 and then tocontract it in a radial direction. The beam hollowing out means includesthe axially symmetrical electrode 38 which is maintained at the samevoltage as the cathode 10 by suitable means, not shown. The electrode 38includes a pointed por tion 42, the point thereof extending upwardly,the portion 42 being supported by spider arms 44 from an annular disk 46comprising another portion of the electrode 38. While only two spiderarms 44 are shown, there may be as many spider arms (three for example)as is desired.

The outer portion of the electrode 38 is turned up and surrounds theouter edges of the disk 34 forming part of the electrode 28. The inneredge of the disk 46 is formed into an upstanding portion 48 whose innershape is cylindrical and whose outer shape is conical and slantsoutwardly in a downward direction. The lower portion of the disk 46 isformed to receive a portion of an electrode 52 as will be explained.Since the electrode 38 is at a high negative potential, that of thecathode l0, and since the electrode 28 is at a high positive potentialwith respect thereto, the portion 36 of the electrode 28 is cut away toprevent arcing between the electrodes 28 and 38. Also the turning up ofthe outer portion of the electrode 38 to surround the electrode 28 is toprevent arcing between the electrode 28 and the microscope casing, notshown, of which the described electron gun may be a part. The shape andposition of the pointed electrode portion 42, and the negative voltageapplied to it, causes the electron beam to be hollowed out to provide asubstantially electron free space within the beam 12 immediatelysurrounding the pointed portion 42, the space so formed beingsymmetrical with respect to the axis of the beam 12.

The means for causing the beam 12 to contract in a radial directionincludes the electrodes 38, 50 and 52. The electrode 50, which is alsoaxially symmetrical, includes an outer annular disk portion 54, atransitional portion 56, spider anns 58 and a central disk portion 60.The transitional portion 56 has a conical upper surface and a moresteeply conical lower surface and joins the lower annular portion 50 tothe spider arms 58 which support the disk 60. While only two arms 58 areshown, there may be as many thereof (three for example), as may bedesired. The electrode 50 is maintained at the same high positivepotential with respect to the cathode l0 as the electrode 28 by suitablemeans, not shown, whereby the arms 58 are also highly positive, andwhereby the electrons of the beam 12 are attracted thereby. To reduceattenuation of the beam 12 by the arms 58, the anns 58 are positioned inthe shadow of the arms 44 which are at the same potential as thecathode.

The electrode 52, which is held at the potential of the cathode 10 bysuitable means, not shown, comprises an annular disk 62 having an innerdiameter equal to the inner diameter of the central upstanding portion48 of the electrode 38 surrounding the beam 12 and an upstandingcylindrical portion 64 having the same inner diameter as the disk 62 anda much smaller outer diameter. The cylindrical portion 64 extends upinto and fits into the electrode 38 and is electrically connectedthereto, to form with the inner surface of the portion 48 a longequipotential cylindrical surface. To provide clearance for the spiderarms 44 and 58, vertical slots 66 are cut in the upper end of thecylinder 64. While no insulation is required for the arms 44, the arms58 and the portion 56 of the electron 50 are insulated by vacuum fromthe cylinder 64. Due to the voltages applied to the electrodes 38, 50and 52, the electrons are attracted inwardly by the positive voltage onthe disk 60, and the electrons are also repelled inwardly by thenegative voltage on the inside surface of the electrodes 38 and 52causing the beam 12 to consolidate and to cross over at the point 68 andto become a solid beam. The electrons of the beam 12 go downward fromthe point 68 so that they appear to originate at the point 70 which isthe point of intersection of straight lines tangent to the envelope ofthe beam. An axially symmetrical electrode 72 is positioned coaxiallywith the electrodes 28, 38, 50 and 52. This electrode 72, which ismaintained at a positive potential of about 560 volts with respect tothe cathode 10 by suitable means, not shown, comprises an outer annulardisk 74 and an inner annular disk 76 connected by a transitional portion78. The disk portion 76 extends up into the lower part of the annulardisk 62 to produce the necessary electronstatic field shape to cause thebeam to appear to come from point 70. The hole in the disk 76 increasesin diameter in a downward direction in a known manner. The beam 12passes through the hole in the portion 76, and the beam emergingtherefrom, due to the electrodes mentioned and to the voltage appliedthereto, appears as if it originated at the point 70 with the sameangular divergence and envelope as if it originated from point 40.However, an ion that originates in the portion of the described gunbelow the ion impermeable disk 60 hits the disk 60 and is stoppedthereby. The ion cannot hit and therefore does not injure the cathode10, whereby the life of the cathode 10 is extended by the provision ofthe described ion trap thereof.

The electron beam is accelerated and further focused by the electrode 72in conjunction with electrodes 80 and 82, whereby the electrode 72 actsboth as an element of the coaxial ion trap described as well as anelement ofa beam accelerator. The axially symmetrical element 80comprises an outer annular disk portion 84 and a centrally positionedupstanding hollow portion 86 having a conical upper surface and acylindrical inner surface. The electrode 80 also includes a downwardlyextending hollow portion 88 having a cylindrical inner surface 90 and acylindrical outer surface 92 adjacent to the disk 84 and an outerconical surface 94 which decreases in diameter in a downward directionjoining the cylindrical surface 92 and 90. The diameter of the surface90 is greater than the inside diameter of the portion 86. An apertureddisk 96 closes the top of the cylindrical surface 90. The aperture inthe disk 96 may be biconical as shown. The disk 96 is thinner than thedisk 84 and the lower surfaces of the disk 84 and 96 are in the sameplane. The upper end of the portion 86 of the electrode 80 extends intothe transitional portion 78 of the electrode 72. The voltage applied tothe electrode 80 by suitable means, not shown, may be 660 voltspositive.

The electrode 82, which is axially symmetrical, has a positive voltageof560 volts applied thereto by suitable means, not shown. This electrode82 comprises an outer annular disk portion 98 having a turned down rim100. The electrode 82 also includes a central annular disk portion 102having a hole therethrough (for the passage of electrons) which hole hasa biconical shape comprising a shorter frustoconical portion, whosediameter decreases in a downward direction, and a longer frustoconicalportion whose diameter increases in a downward direction, the twosmaller diameter portions of the frustrums being joined. The shape ofthe aperture through the disk portion 102 aids in maintaining focus ofthe beam 12. The electrode 82 also includes a transitional portion 104having an upper conical surface conforming in angle with the surface 94of the electrode 80 and spaced therefrom, the conical surface 94extending into the transitional portion 104. The outer surface of thetransitional portion 104 is cylindrical. The portion 106 of theelectrode 82, which, as shown, may be made up as a separate part fromthe portions 98, 100 and 104 thereof for the purpose of ease ofmanufacture, fits over the lower end of the cylindrical outer surface ofthe transitional portion 104. The portion 106 has a conical innersurface 108 whose diameter decreases in a downward direction. Anapertured plate 1 l closes the lower part of the lower portion 106. Theaperture in the plate 110 may have a wedge-shaped inner edge as shown.X-deflecting plates 112 and Y-deflecting plates 114, only one of theY-plates being shown, are provided in the chamber within the conicalsurface 108 and between the apertured disks 102 and 110. By adjustmentof potentials provided by suitable means, not shown, applied to thedeflection plates X and Y, the direction of the beam 12 emerging belowthe disk may be controlled.

The device so far described may be used as a complete electron gunrequiring no additional acceleration and including the described iontrap. However, the described gun may also be used as a source of a beamof accelerated electrons subject to further acceleration, as in anelectron microscope. When so used, a further acceleration electrode 120may be provided. This electrode 120 comprises a disk apertured forpassage of the electron beam 12 therethrough, to which high positivevoltage in the order of 6.6 to 33kv. may be applied. This acceleratingelectrode 120 may be one of a plurality (up to 15) of similar electrodespositioned along the beam 12 below the electrode 120 to whichsuccessively higher positive voltages are applied, the highest of whichmay be as high as 500 kv. when the voltage on the electrode 120 is 33kv. The voltage on the accelerating electrodes, of which only theelectrode 120 is shown, while accelerating the beams 12 passingtherethrough also produces an electron lens action which results in adefocusing of the beam 12. Using the described electron gun withaccelerating electrodes such as electrode 120, only the voltage on thegrid cap 14 need be changed to retain focusing of the beam 12, as thevoltage of the accelerating electrodes such as electrode 120 is changed.This is due to the fact that the accelerating electrode 120 and anysubsequent accelerating electrodes such as electrode 120 (not shown)constitute a weak electrostatic aperture lens having a very long focallength. This long focal length extends in both directions from theelectrode 120. The electrodes 72, and 82, on the other hand, constitutea much stronger electrostatic lens having a much shorter focal length.Because of the proximity of this strong lens comprising the electrodes72, 80 and 82 to the electrode and to the subsequent acceleratingelectrodes (not shown), the strong lens 72, 80 and 82 is immersed in thefocal length of the weak lens comprising the electrode 120 andsubsequent electrodes (not shown). In this situation a small focusingchange in the stronger lens 72, 80 and 82 can overcome a focusing changedue to a great voltage change in the weaker lens. The small focusingchange in the strong lens 72, 80 and 82 is provided by varying thevoltage in the grid cap 14, without varying the voltages in theelectrodes 70, 80 and 82. Since change of the voltage on the grid cap 14changes the position of the point 70, where the electrons of the beam 12appear to originate, the focusing effect on the beam of electrons of thelens 72, 80 and 82 is changed.

This change of potential on the grid cap 14 is from about 45 volts toabout 60 volts, or about 33 percent, as the voltage on the electrode 120is changed from about 6.6 kv. to about 33 kv. or about 500 percent.Therefore, the electron gun described minimized the defocusing of thebeam thereof upon further acceleration of the beam and reduces thechanges in potential of the grid cap 14 necessary to track, that is tokeep the beam focused, as the acceleration voltage on the electrodes,such as electrode 120, is changed.

While the several electrodes (except the cathode which may be made oftungsten) may be made of any solid nonmagnetic conductor, preferablythey may be made of nonmagnetic stainless steel.

What is claimed is:

1. An electron gun of particular utility in the environment of anelectron microscope comprising, in combination,

means to produce an electron beam;

a first accelerating lens of a relatively short focal length sopositioned that the beam passes therethrough, and at a positivepotential relative to said means to produce an electron beam;

a second accelerating lens of a relatively long focal length comparedwith the focal length of said first lens and so positioned that the beampasses through the second lens after it passes through the first lens,

said second lens being at a potential significantly more positive thansaid first lens,

said second lens being spaced from said first lens by a distance lessthan the focal length of the second lens,

whereby defocusing of said beam as a result of a significant change inthe potential applied to said second lens is readily corrected by arelatively small change in the potential difference between said beamproducing means and said first lens. 2. An electron gun according toclaim 1 wherein said first accelerating lens comprises a plurality ofelectrodes spaced from each other and each at a potential significantlylower than the potential of said second accelerating lens. 3. Anelectron gun according to claim 2 wherein said means to produce anelectron beam includes a cathode; and a grid cap in proximity to saidcathode and having an opening therethrough for passage of electrons fromsaid cathode; said grid cap being at a potential negative with respectto the potential of said first lens, and said grid cap potential beingadjustable to provide said relatively small change in the potentialdifference between said beam producing means and said first lens. 4. Anelectron gun according to claim 1 which further includes means todeflect said electron beam, said means to deflect being positionedbetween said first accelerating lens and said second accelerating lens.5. An electron gun according to claim 4 wherein said means to produce anelectron beam includes a cathode; and a grid cap in proximity to saidcathode and having an opening therethrough for passage of electrons fromsaid cathode; and an ion trap between said means to produce the electronbeam and said first accelerating lens. 6. An electron gun according toclaim 5 which further includes additional lens means extending aroundsaid ion trap and between said means to produce the electron beam andsaid first accelerating lens.

1. An electron gun of particular utility in the environment of anelectron microscope comprising, in combination, means to produce anelectron beam; a first accelerating lens of a relatively short focallength so positioned that the beam passes therethrough, and at apositive potential relative to said means to produce an electron beam; asecond accelerating lens of a relatively long focal length compared withthe focal length of said first lens and so positioned that the beampasses through the second lens after it passes through the first lens,said second lens being at a potential significantly more positive thansaid first lens, said second lens being spaced from said first lens by adistance less than the focal length of the second lens, wherebydefocusing of said beam as a result of a significant change in thepotential applied to said second lens is readily corrected by arelatively small change in the potential difference between said beamproducing means and said first lens.
 2. An electron gun according toclaim 1 wherein said first accelerating lens comprises a plurality ofelectrodes spaced from each other and each at a potential significantlylower than the potential of said second accelerating lens.
 3. Anelectron gun according to claim 2 wherein said means to produce anelectron beam includes a cathode; and a grid cap in proximity to saidcathode and having an opening therethrough for passage of electrons fromsaid cathode; said grid cap being at a potential negative with respectto the potential of said first lens, and said grid cap potential beingadjustable to provide said relatively small change in the potentialdifference between said beam producing means and said first lens.
 4. Anelectron gun according to claim 1 which further includes means todeflect said electron beam, said means to deflect being positionedbetween said first accelerating lens and said second accelerating lens.5. An electron gun according to claim 4 wherein said means to produce anelectron beam includes a cathode; and a grid cap in proximity to saidcathode and having an opening therethrough for passage of electrons fromsaid cathode; and an ion trap between said means to produce the electronbeam and said first accelerating lens.
 6. An electron gun according toclaim 5 which further includes additional lens means extending aroundsaid ion trap and between said means to produce the electron beam andsaid first accelerating lens.